fix(pod): restore --stop-after and --terminate-after, validated

[justinwlin]

Brings back --stop-after and --terminate-after, removed in #330, with every client-side way of losing the deadline closed.

Do not merge until the API enforces the timer. Until then #330's removal is the correct behaviour, and this PR would put the silent-billing failure straight back. Stacked on #330.

What this fixes

Problem	Fix
Docs advertise 1h, 24h, 7d; the GraphQL DateTime scalar rejects them	duration.ParseDeadline resolves durations against the clock and sends RFC 3339
Raw GraphQL scalar error leaked to the user	Rejected locally, before the create call, so a bad value never costs a billed pod
Past timestamps accepted, deadline never arrives	Rejected, with the current time in the message
CPU pods accept the flag and silently drop it (REST v2 has no scheduling field)	Errors instead
No confirmation the timer was applied	Resolved deadline echoed to stderr

$ runpodctl pod create --gpu-id "NVIDIA RTX 4000 Ada Generation" --image ubuntu:22.04 --stop-after 4m
note: auto-stop scheduled for 2026-08-26T16:48:35Z
{ "id": "bdwcl5lzqwqny0", ... }

What it does not fix

Whether the pod actually stops. stopAfter / terminateAfter are write-only from a client: valid input on PodFindAndDeployOnDemandInput, not readable on the Pod type, absent from REST v2. The CLI cannot verify the deadline landed, so the stderr note describes what was sent, not what will happen. Enforcement is an API-side change.

Verification

Live against production:

$ runpodctl pod create --compute-type CPU --image ubuntu:22.04 --stop-after 2h
{"error":"--stop-after is not supported for compute type CPU; ...","code":"cli_error"}

$ runpodctl pod create --gpu-id "NVIDIA A40" --image ubuntu:22.04 --stop-after "1h ago"
{"error":"invalid --stop-after: invalid time \"1h ago\": use a duration like 2h, 7d or an rfc3339 timestamp ...","code":"cli_error"}

$ runpodctl pod create --gpu-id "NVIDIA A40" --image ubuntu:22.04 --terminate-after "2020-01-01T00:00:00Z"
{"error":"invalid --terminate-after: ... must be in the future (now is 2026-08-26T16:44:26Z)","code":"cli_error"}

Unit tests: TestResolvePodSchedule, TestResolvePodScheduleNotesTheDeadline, TestParseDeadline. go test ./... green.